Where to enter it
At checkout, before you pay. There is a field for a code on the payment step. Enter it, apply it, and check the total changes before you complete the order.
Enter it exactly
Codes are usually not case sensitive, but they are sensitive to stray spaces, which is easy to pick up when copying from a message. Paste it and then check for a space at either end.
After you have paid
A code cannot be applied to a completed order. If you forgot, contact us before you use the eSIM and we will see what we can do rather than leaving you out of pocket.
Referral codes
A referral code is entered the same way. The person who referred you receives their reward once your order completes. See "My referral credit has not arrived".
If it is rejected
See "My promo code will not apply", which covers the usual reasons.